Some smarty folks at the University of Pennsylvania have developed a backpack that helps you walk while also turning your movement into energy.

The backpack's load plate is mounted to the pack and suspended by springs, which allows the load to echo the hip's movement. This drives a rack-and-pinion device, which in turn powers a motor that acts as a generator. The result is a conversion of the body's mechanical energy to electricity.

This could definitely be practical for explorers, hikers, soldiers, etc. Although it could be a bit overkill for Timmy on his first day of kindergarten. Stick to the Spongebob kids, no need to save the earth yet.